1. Field of the Invention
This invention relates to an image reading device and an image forming device which detect a document size using a line sensor for reading a document, instead of reflected type sensors arranged in the positions corresponding to various sizes of standard-sized documents.
2. Description of the Related Art
An image reading device, such as an image scanner, has a function which detects automatically the size of a document laid on a transparent document base, such as a contact glass. The commonly used method of detecting document size is that reflected type sensors are arranged in the positions corresponding to various sizes of standard-sized documents, and the size of a document is determined based on a combination of output signals from the sensors.
In recent years, however, for the purpose of cost reduction, a new method is adopted, and the new method is that the size of a document is detected using a line sensor for reading the document, rather than using the reflected type sensors mentioned above.
For example, Japanese Laid-Open Patent Application No. 2007-158965 discloses an image reading device in which only predetermined positions of a document placed on the contact glass are read, a black/white judgment for image data of a plurality of patches (rectangle regions) corresponding to the predetermined positions of the document is performed using the read image data of the document, and the size of the document is judged based on the results of the black/white judgment of the respective patches.
In the image reading device of Japanese Laid-Open Patent Application No. 2007-158965, the document size judgment process is performed in this way. Because it is no longer necessary to scan the whole surface of the document in order for judging the document size, the time needed for judging the document size can be shortened.
However, the document size judgment as in Japanese Laid-Open Patent Application No. 2007-158965 has a problem in that, if the pressure plate is already closed when reading the rectangle regions for document size detection, erroneous detection occurs. This is because the back surface of the pressure plate holding is optically read by the line sensor.
An approximate minimum time for the user to close the pressure plate can be predicted. However, there may be a case in which the user closes the pressure plate earlier than the predicted time. A conceivable method for preventing the problem is to shorten the time of reading of the rectangle regions for document size judgment. However, if the amount of data being read is decreased, the result of document size judgment is easily influenced by the surrounding lighting or others, which may be the factor of erroneous judgment. Thus, it is necessary to make the time of reading of the rectangle regions as long as possible and average the read image data, in order to reduce the influences of the surrounding lighting etc.
Usually, the reading of document detection is started in response to a signal output from the photo sensor which has detected an inclined state of the pressure plate in which a certain angle is made by the contact glass and the pressure plate. For this reason, it is necessary to complete the reading of the rectangle regions for document size judgment during a short period from the start timing until the pressure plate is completely closed.
However, the smaller the angle between the contact glass and the pressure plate, the larger the influence of the white sheet on the back of the pressure plate. Then, the possibility of erroneous detection becomes higher.
Moreover, the influence of the light source lamp of the image reading unit may be another problem of erroneous detection. For example, when the temperature of the environment where the image forming device is placed is low, the lighting up of the lamp may delay. Or there may be a case in which the lamp stops lighting up due to the deterioration or other factors. In such cases, erroneous detection of the document size may occur.
Accordingly, the document size judgment as in Japanese Laid-Open Patent Application No. 2007-158965 has the problem in that the user cannot recognize erroneous detection of the document size which may occur when the pressure plate is closed too early or when the lamp is not switched on normally.